Zn-binding motifs, ubiquitous among gene-regulatory factors, provide an economical mechanism of metal-dependent protein folding. In earlier studies we described a novel Zn-binding motif in the nucleic-acid binding domain of human transcriptional elongation factor TFIIS. The NMR structure consists of a three-stranded antiparallel ?-sheet designated the Zn ribbon. Analysis of the sequence database suggested a conserved family of Zn-ribbon motifs in a diverse family of nucleic-acid-binding proteins, including RNA polymerase itself. The intriguing discovery of a putative Zn ribbon sequence in an archeal genome (Kaine, B.P., Mehr, I.J., & Woese, C.R. (1994) Proc. Natl. Acad. Sci. USA 91, 3854-8) now permits this hypothesis to be tested directed in a system of exception thermal stability. The archeal gene encodes a protein homologous to a subunit of human RNA polymerase II. We propose to determine the solution structure of this RNA polymerase subunit by multidimensional heteronuclear NMR methods.